AC-AC converter with high frequency link

ABSTRACT

An AC-AC Converter for an AC source which in one embodiment has a first rectifier section rectifying the AC source into a first pulsed DC link voltage signal and a high frequency modulating section coupled to the first pulsed DC link voltage signal and producing a high frequency AC voltage signal. A high frequency transformer is coupled to the high frequency AC voltage signal producing a transformed high frequency AC signal. There is a second rectifier section coupled to the transformed high frequency AC signal and producing a second pulsed DC voltage signal and an unfolder section coupled to the second pulsed DC voltage signal and producing an output AC signal.

STATEMENT OF GOVERNMENT INTEREST

Portions of this invention were made with government support undercontract number N0001407C0415 awarded by Office of Naval Research (ONR).The government has certain rights in the invention.

BACKGROUND

Power transmission from a source to a load typically requires multiplepower conversion stages such as from high voltage AC source to a lowervoltage AC at the loads. Many modern power systems require large andheavy conventional transformers. The weight and volume of thesetransformers is a barrier to the development of expanded electricalcapabilities associated with certain power system applications.

This is especially relevant with the 50 Hz and 60 Hz transformers usedfor many utility distribution systems as well as the electricaldistribution systems associated with ships, planes, and railroads. Thesize of the present conversion systems impacts the placement criteriathat can be problematic in space-restricted areas. The weight of thetransformers not only impacts the logistics, but also impacts theoperational efficiency if the heavy transformers are carried onboard.

Recent developments and designs indicate that high frequency solid stateor electronic transformers are enhanced replacements for bulkyline-frequency iron core transformers. High frequency switched powerelectronic transformers bring about significant reductions in size andweight compared to conventional line-frequency transformers. Inaddition, controls can be embedded to provide enhanced functionalitysuch as fault current limiting and improved power quality.

One known approach uses rectification of the incoming AC source to a DCsignal followed by a high frequency isolated DC-DC converter and theninversion back to the output AC signal. There can be multiple steps toconvert the power source to the load bus. The multiple switching stagesgenerally lead to higher losses and system cost. Another approachdirectly converts the incoming low frequency AC to high frequency AC,which is then fed to a transformer. The transformer secondary voltage isthen reconstructed to a low frequency output. One of the disadvantageswith this type of circuit design is the use of bidirectionalsemiconductor switches.

While there have been attempts to configure AC-AC converters used forvoltage and frequency conversion, there remains continued needs for moreefficient designs with improved performance.

SUMMARY OF THE INVENTION

The invention relates generally to power distribution electronics andmore particularly to electronic transformers and AC-AC convertersystems.

One embodiment is an AC-AC converter for a high voltage AC source,having a first rectifier section rectifying the AC source into a firstpulsed DC link voltage signal with a high frequency modulating sectioncoupled to the first pulsed DC link signal and producing a highfrequency AC voltage signal. There is a high frequency transformercoupled to the high frequency AC voltage signal producing a transformedhigh frequency AC signal and a second rectifier section coupled to thetransformed high frequency AC signal and producing a second pulsed DCvoltage signal. The unfolder section is coupled to the second pulsed DCvoltage signal and produces an output AC signal. A filter may be coupledin between the second rectifier section and unfolder section on the DCport and/or after the unfolder section at the AC port to remove highfrequency content from the AC output voltage. Filters can also be placedon the input and/or output.

The AC-AC converter may further comprise a reactive converter sectionsupplying a reactive current to the low voltage AC signal and thereactive converter section can be coupled to the low voltage AC signalby one of a parallel and series connection.

A further feature includes wherein the first and second rectifiersections include antiparallel switches that provide bidirectional andreactive power flow.

Another aspect of the AC-AC converter includes having a singleelectronic transformer cell. One embodiment includes having a singlephase AC-AC converter or solid state power system that includes at leastone additional series coupled or parallel coupled electronic transformercell. Yet a further aspect is a three phase AC-AC converter or solidstate power system that includes three single phase electronictransformer cells configured in delta or wye in the primary orsecondary.

A further feature includes having at least one capacitor coupled betweenthe first rectifier section and the high frequency modulating sectionfor filtering high frequency components.

The high frequency modulating section in another embodiment has at leasttwo series coupled transistors to each switch position of the highfrequency transformer wherein at least one of the series coupledtransistors can have different voltage ratings. One feature of theseries coupled transistors is that they can be switched such that atleast one of the transistors is inactive for at least some period oftime. In other words, all the transistors do not need to be switchedsimultaneously. Furthermore, the series coupled transistors can beswitched according to a threshold level such that only one of thetransistors is active during a lower voltage region, only one of thetransistors is active during a middle voltage range, and/or all of thetransistors are active during peak voltages. A further feature is thatthe transistors active during the lower voltage region is a lower ratedtransistor with respect to the transistor active during the middlevoltage region. Yet another aspect is that the switching is based upon ameasured voltage range and/or a calculated voltage range.

One embodiment is an AC-AC converter, comprising a first rectifiersection rectifying an AC source into a first pulsed DC link signal,wherein the inverter section is coupled to the first pulsed DC linksignal and produces a high frequency AC signal, and wherein the invertersection has at least two transistor banks. There is a high frequencytransformer coupled to the high frequency AC signal that produces atransformed AC signal, with a second rectifier section coupled to thetransformed AC signal and producing a second pulsed DC link signal. Anunfolder section is coupled to the first pulsed DC link signal andproduces an AC signal output.

A feature of this system is wherein each bank of transistors comprisesat least two transistors coupled in series. Another aspect is that atleast one of the transistors has a different voltage rating. Yet afurther feature includes wherein the transistors in each of thetransistor bank are switched such that at least one of the transistorsis inactive for at least some period of time. One aspect is that thetransistors are switched according to a switching frequency and whereina lower switching frequency is used for lower DC link signals and ahigher switching frequency is used for higher DC link signals. A furtherfeature is that the transformer peak flux level is held constant.

Yet another embodiment is a three phase AC-AC converter having threeelectronic transformers coupled together to convert a three phase ACsignal input to a three phase AC signal output. Each of the electronictransformers can include a first rectifier section rectifying the ACsignal into a first pulsed DC link signal, wherein the inverter sectionis coupled to the first pulsed DC link signal and produces a highfrequency AC signal. A high frequency transformer is coupled to the highfrequency AC signal and produces a transformed AC signal with a secondrectifier section coupled to the transformed AC signal and produces asecond pulsed DC link signal. There is an unfolder section coupled tothe second pulsed DC link signal and produces the three phase AC signaloutput having a frequency and shape similar to the three phase AC signalinput.

One of these features includes wherein each of the electronictransformers produces a phase current that is used to generate a linecurrent, wherein each phase current combines a harmonic componentaltering the phase current, and wherein the line current remains thesame. Another aspect is that the electronic transformers can be coupledto form one of a delta and wye connection on at least one of a primaryand a secondary.

One embodiment is for a method for converting AC power using a singlephase AC-AC converter section including receiving an input AC signal,rectifying the AC signal into a first pulsed DC link signal, modulatingthe first pulsed DC link signal into a high frequency link, transformingthe high frequency link to a transformed voltage link, rectifying thetransformed voltage link into a second pulsed DC link signal, andunfolding the second pulsed DC link signal to an output AC signal.

An additional feature of the method further comprises introducingreactive currents at the output AC signal. The method further comprisesproviding bidirectional and reactive power flow.

Another aspect includes stacking at least two of the three-phase AC-ACconverter sections to provide for a three-phase AC source.

An additional embodiment includes modulating the rectified AC signalthat includes switching among at least two transistors coupled to eachswitch position of a high frequency transformer. One switching featureuses a lower rated transistor for lower voltages. Another featuresincludes modulating the rectified AC signal wherein the switching isamong at least one transistor coupled to each switch position of a highfrequency transformer, and wherein the switching frequency is a variableswitching frequency. Yet a further feature is that variable switchingfrequency establishes a substantially constant magnetic flux density.

In one embodiment, the AC signal is rectified to a pulsed DC link thatis then converted to a high frequency AC signal. The high frequency ACis transformer isolated and rectified to provide a pulsed DC at thesecondary. This is then unfolded to an AC voltage of the same frequencyand shape as the incoming AC. An optional reactive current converter canbe coupled to the output to feed inductive loads since the main circuitdoes not typically handle reactive currents due to diode rectification.

The features and advantages described herein are not all-inclusive and,in particular, many additional features and advantages will be apparentto one of ordinary skill in the art in view of the drawings,specification, and claims. Moreover, it should be noted that thelanguage used in the specification has been principally selected forreadability and instructional purposes, and not to limit the scope ofthe inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a basic block diagram illustrating an AC-AC converter systemconfigured according to one embodiment.

FIG. 2 is a schematic diagram showing an electronic transformer cellconfigured in accordance with one embodiment.

FIG. 3 a is a partial schematic diagram configured in accordance withone embodiment.

FIG. 3 b shows the pulsing DC link signal and the high frequency linksignal.

FIG. 4 is a schematic diagram showing an electronic transformer cellwith reactive compensation configured in accordance with one embodiment.

FIG. 5 is a schematic diagram illustrating an electronic transformercell with bidirectional power flow configured in accordance with oneembodiment.

FIGS. 6 a and 6 b shows a single phase electronic transformer withstacked electronic transformer cells with series coupling at primaryside and parallel coupling at secondary side according to oneembodiment.

FIG. 7 a is a schematic diagram illustrating a three-phase electronictransformer with three electronic transformer cells in accordance withone embodiment.

FIG. 7 b shows a series of waveforms illustrating the signaltransformations according to one embodiment.

FIG. 7 c is another schematic diagram illustrating a three-phase AC-ACconverter system in accordance with one embodiment.

FIG. 8 a-8 c shows prior art signal waveforms of a constant switchingfrequency configuration.

FIG. 9 a-9 c shows signal waveforms for a variable switching frequencyconfiguration according to one embodiment.

FIG. 10 a shows prior art phase current waveforms.

FIG. 10 b illustrates a diagrammatic perspective of the phase currentcalculations for the line currents for a delta-connected transformer.

FIG. 10 c shows the prior art line current waveforms.

FIG. 10 d shows a primary phase current waveform in accordance with oneembodiment.

FIG. 10 e shows an additional component to be combined with the primaryphase current waveform of FIG. 10 d in accordance with one embodiment.

FIG. 10 f shows the phase current waveforms for the manipulated internalwaveforms in accordance with one embodiment.

FIG. 11 is a flow chart illustrating the process flow for the AC-ACconversion system in accordance with one embodiment.

DETAILED DESCRIPTION

In general, certain embodiments of the system and techniques detailedherein provide for more efficient power conversion and in particular forAC-AC conversion. A technical effect of the system and methods herein isto provide an AC-AC converter having a high frequency link. Someadvantages of the system and techniques include minimizing the number ofswitching devices, higher efficiency, lowering weight, reducing the needfor sensors and controls, and automatic voltage sharing betweenseries-connected modules. The systems and techniques herein provide newapproaches to implementing solid state electronic transformer cells withminimized use of high frequency switching stages, optimized usage ofcomponents, and improved efficiencies.

One embodiment of an AC-AC conversion system 10 is shown in FIG. 1.There is some AC source 20 such as a single phase or multi-phase highvoltage AC signal. There can be an input filter section (not shown) tofilter the AC source 20 and condition the signal. The AC source 20 iscoupled to a rectifier section 30. In this example, the rectifiersection 30 is a diode rectifier that converts the AC signal into apulsed DC link at twice the AC source frequency.

An inverter section 40 performs high frequency modulation of therectified signal into a high frequency (HF) link. According to oneembodiment the inverter section 40 contains a plurality of transistorsthat are switched via gate drivers and a control section to provide thehigh frequency link signal. The HF link signal is input to a highfrequency transformer 50 that transforms the voltage level of the HFlink according to the design criteria of the transformer 50 for theintended application. In this example, the high frequency link signal istransformer isolated and rectified to provide a pulsed DC at thesecondary. The transformer 50 can be used to change the voltage levelsbetween the AC source 20 and the AC load 80 such as by employingstep-down transformer or step-up transformer, and also can provideapproximately the same voltage level such as a unity transformer. Othertransformer configurations known in the art can be integrated employingthe techniques herein.

The transformed voltage signal is input to a rectifier and unfoldersection 60 that rectify the HF link transformed signal and then unfoldsthe rectified signal to an AC signal coupled to an AC load 80, and inthis example, the unfolded signal has an AC voltage of the samefrequency and shape as the incoming AC source. A small filter isoptionally coupled either in between the second rectifier section andunfolder section on the DC port or after the unfolder section at the ACport to remove high frequency content from the AC output voltage.

An optional reactive current converter 70 can be coupled to therectified AC signal to feed inductive loads, if the main circuit doesnot process the reactive currents.

According to one embodiment, the rectifier 30, high frequency modulator40, high frequency transformer 50, and the rectifier/unfolder 60 form anelectrical equivalent of a conventional transformer such that the solidstate electronic system 10 can replace the conventional transformer.

There are many alternate embodiments incorporating the system andtechniques described herein. For example, one embodiment includes thereplacement of the large and heavy conventional transformers, such as 60Hz transformers, that use an intermediate high frequency switchmodetransformer with the AC-AC converters and electronic transformer cellsdetailed herein thereby reducing size and weight.

While the power is processed through multiple stages, in one embodimenttwo of the conversion stages, namely the rectifier section 30 and therectifier/unfolder 60, are highly efficient as they only switch at thelow AC source frequency. In addition, since control requirements areminimal in these two stages, the system reliability is improved.

One application for the described system is to couple one or moresection having switches and/or devices in series to handle the pulsed DClink. Such a coupling enables switching of the series-connected devicesaccording to the instantaneous voltage of the pulsed DC link. Forexample, for 1.7 kV and 3.3 kV devices that are series-connected tobuild a 5 kV device, only the 1.7 kV device needs to be switched duringthe period when pulsed DC link voltage is below ˜1200V. This providesfor a more efficient system operation.

There are several embodiments incorporating unique elements into thegeneral structure of FIG. 1 such that the operational performance isenhanced and efficiency improved as detailed herein.

Referring to FIG. 2, a more detailed schematic presentation of one ofthe AC-AC electronic transformers is shown. An AC source 210 is an inputto the system 200. The AC source can be an AC signal 215 having avoltage level and frequency with corresponding appropriate componentsaccording to the design criteria. For illustrative purposes, the ACsignal can be a three-phase, 13.8 kV, 60 Hz signal such as used on-boardcertain shipping vessels, however in this example it is a single phase.While this example illustrates converting a higher input voltage to alower output voltage, other voltage conversions are within the scope ofthe invention.

The input AC signal 215 is rectified by the rectifier section 220 andoutputs a rectified or pulsed DC link signal 225. The capacitor C1provides some filtering and snubbing. The capacitor C1 according to oneembodiment has a low capacitance value as compared to the typical bulkcapacitors to allow the DC bus to be discharged during the cycle inorder to enable the pulsed rectified signal 225. By way of example, ifC1 had a high capacitance value, the pulsed rectified signal 225 wouldnot be fully discharged to zero in the cycle. Thus in this example, thecapacitance value of C1 is low enough to allow the peak value to fullydischarged by the load, and the pulsed signal 225 results. Therectification of the high voltage AC signal to high voltage links can beaccomplished in several ways such as by silicon controlled rectifiers(SCRs) or a diode bridge. By way of example, for a 60 Hz high voltageinput source, an example of diodes includes 60 Hz silicon high voltagediodes.

The rectified signal 225 is an input to the inverter section 230 thatperforms high frequency modulation and generates a high frequency linksignal 235. The modulated signal has an envelope and pulse widthaccording to typical pulse width modulation techniques. The invertersection 230 can be, for example, MOSFET H-bridges inverting the highvoltage pulsed DC link also referred to as rectified signal 225 to highfrequency AC pulse width modulated signals. The inverter section 230 canbe various switching devices such as SiC MOSFET. In a further example,the high frequency link signal can be modulated at 20 kHz by an H bridgechopper circuit, which can include silicon carbide components.

A high frequency transformer 240 is coupled to the high frequency linkssignal 235 transforming the voltage levels between the primary andsecondary according to the transformer design. In this design, the highfrequency links signal is stepped down at the secondary side of thetransformer 240 which outputs a transformed high frequency AC signal andis an input to the voltage rectifier section 250 producing a rectifiedlower voltage or second pulsed DC link signal 255.

The lower voltage rectified signal is then unfolded via the unfoldersection 260 to produce the stepped down AC voltage 275 for the bus orload 270. According to one embodiment the unfolder section is atransistorized H-bridge such as Si MOSFETs.

Capacitors C2 provide some high frequency filtering and snubbing similarto C1 however these capacitors in one aspect are not the typical bulkcapacitors used in DC bus circuits.

It should be readily apparent that the output voltage level and outputfrequency depend upon the input signal and the design of the system.There are a number of variations and further features that can beincorporated in the present designs.

Referring to FIG. 3 a and FIG. 3 b, one embodiment relates tocontrolling transistors, such as IGBTs, in the inverter/converter inorder to handle higher voltages and/or provide for higher switchingrates. There are designs in which a single high rated transistor wouldnot be practical or would impart drawbacks. In such applications, twotransistors can be coupled in series thereby splitting the high voltagerequirements required for a single transistor allowing lower ratedtransistors.

For example, it is known that for certain high voltage inverterapplications in which a single transistor is not able to handle thepulse peak, another transistor can be coupled in series to share theload requirements. The two transistors coupled in series are typicallysynchronized to switch in unison and share the load equally between thetransistors. These series coupled transistors are used in manyimplementations, and traditionally the series coupled transistors aresynchronized to balance the load between the transistors. For example,with two transistors in series, the active electronics in conjunctionwith the gate drivers ensure that both transistors are switched On andOff in a manner so the total voltage is split equally across the twotransistors.

As shown in FIG. 2 and FIG. 3 b for the high voltage rectifier link 390,the high voltage DC link rectified value is not constant but ratherchanges over time 350, 360 such that the instantaneous voltage levelgoes from a low value 350 to the full load value 360 and then back downto the low level. The DC pulsating link signal 395 therefore goesthrough instances of higher instantaneous voltage, wherein theinverter/converter is designed to handle the highest instantaneous valueand periods of much lower instantaneous values where there is amplemargin for the voltage requirements.

Some of the distinguishing features of the present system include usingtransistors with lower voltage ratings during lower voltage periodswherein the load is not shared among all the transistors. Anotherfeature involves switching of the transistors such as according to thevoltage level and may be for less than all the transistors. Thesefeatures can be used, for example, within the inverter section 230 ofFIG. 2.

Referring again to FIGS. 3 a and 3 b, two or more transistors 310, 320are coupled in series to form a series bank of capacitors 330. Oneadvantage of a series bank of transistors 330 is the ability to handlehigher voltages. As noted herein, the inverter section 370 operates togenerate the HF link signal that is an input to the transformer 380.

For example, to handle 10 kV, each individual transistor 310, 320 couldbe switched to handle a portion of the load and collectively handle theentire 10 kV. In one example, each transistor 310, 320 could split thevoltage and each handle the same voltage, wherein in the 10 kV exampleeach transistor 310, 320 could be rated at 5 kV. One 5 kV transistorwould be turned on during the low voltage stage until the voltage levelapproached the 5 kV level and then the second 5 kV transistor would beturned on to handle the higher voltage levels. In another example, threetransistors in the transistor bank 330 could each have a 3.3 kV or 4 kVrating. In yet another example the transistor bank 330 can havetransistors with differing voltage ratings but with a total capabilityto meet or exceed the load requirements. This switching approach iscontrary to the conventional approach that typically switches thetransistors in synchronism to share/balance the voltage in thetransistors.

The threshold margins for switching can be designed according to certaindesign criteria and applications. For safety purposes, the transistorswitching would typically be turned on prior to overloading a singletransistor and in one embodiment the switching occurs when the voltagelevel reaches about 60% of the maximum value for a particulartransistor. Likewise, the switching off of the transistors can also usea derating or safety margin. One example of the switching section (notshown) is to sense the voltage level and switch when a certain level wasobtained. Another example computes the voltage level and still anotherexample combines some sensing and some computing to derive the dynamicvoltage level. The switching itself generally depends upon the gatedriver and the active switching electronics.

Another feature of this system is to use less transistors and/ortransistors with a lower voltage rating within the transistor bank 330for periods having lower instantaneous voltages 350 thereby having anunbalanced or unequal split between all the transistors in thetransistor bank 330. The switching control would add additionaltransistors as needed to handle the higher instantaneous voltages 360.For example, during the lower voltage operation 350, less transistors orlow rated transistors in the transistor bank 330 are required to handlethe instantaneous load 350. As the load increases 360, additionaltransistors and/or transistors with higher ratings can be used to handlethe load. There are many benefits for the techniques detailed herein,including handling larger voltages with smaller and less expensive lowerrated transistors. A further aspect relates to the faster switching ofthe smaller transistors and in particular to a switching scheme that canlimit the number of transistors involved in the operation for lowervoltages.

In one example of the present design, since there are periods of lowvoltage, less than the total number of transistors can be used duringthe periods of low voltage. In one example, the system would sense thevoltage and if limits are between the range that can be handled by asingle transistor then only the single transistor would be employedthroughout the low voltage state. The switching and control of thetransistors would depend on the gate driver and the active electronics.The load would not be balanced across other transistors during this lowvoltage period.

As an illustrative example, assuming the load requirements are 10 kV,and that only two IGBTs are in the transistor bank 330, namely a firsttransistor 310 rated at 6.5 kV and a second transistor 320 rated at 3.5kV. The 6.5 kV transistor and 3.5 kV transistor are deployed in series,wherein during the lower voltage periods only the single 3.5 kVtransistor is used. The single lower rated transistor 320 allows forfaster switching and lower switching losses during the period when thesecond transistor 310 is not exercised.

In more particular detail, during the lower voltage levels 350, thelower rated transistor 320 is actively switched and as the voltage loadincreases, the 3.5 kV transistor 320 can be turned Off and the 6.5 kVtransistor 310 can be turned On. If the safety margin or threshold wasset at 75% of peak for the 3.5 kV transistor, the switching to the 6.5kV transistor would occur when the voltage rose to 2.625 kV. As thevoltage continues to increase, the 3.5 kV transistor 320 can be turnedOn to accommodate the full voltage. Similarly, as the voltage leveldrops, the 3.5 kV transistor 320 can be turned Off as the voltage startsto decrease. The 6.5 kV transistor 310 can then be turned Off while the3.5 kV can be turned On at the lower levels.

The embodiment of FIG. 4 is similar to that of FIG. 2, with the additionof a reactive power section 470. The AC source 410 is rectified by thefirst rectifier section 420 and the rectified signal is then modulatedby the modulator section 430 to produce the high frequency link signalthat is then introduced to the primary of the high frequency transformer440 that steps down the voltage at the output of the secondary of thetransformer 440. The high frequency lower voltage signal is rectified bythe second rectifier section 450 and the rectified signal is thenunfolded by the unfolder section 460 to generate the real current ACsignal delivered to the load 480. Filtering of high frequency componentscan be accomplished by using capacitors C1 and C2.

The AC signal from the unfolder 460 represents the real current signalas the reactive component does not get processed therein. In order tocompensate for the reactive component, a reactive processing section 470is used to process the reactive current that is then coupled to the realcurrent AC signal to produce the load current output AC signal 480. Thereactive processing section 470 can be coupled in series or parallel andsupplying inductive loads.

FIG. 5 illustrates one embodiment for an AC-AC conversion system that issimilar to the unidirectional power flow of FIG. 2 but also providesbidirectional/reactive power flow. In one example, the bidirectionalpower flow can be introduced by placing antiparallel switches in therectifier sections. In this example, the first rectifier section 520employs high voltage low frequency switches such as silicon IntegratedGate Commutated Thyristors (IGCTs) or insulated gate bipolar transistors(IGBTs) across the diodes or SCRs.

For illustrative purposes, in one example, the input signal 510 isrectified by the antiparallel switches in an inverter section thatproduce the DC link voltage signal that is high frequency modulated bythe modulator section 530. In one aspect, to provide the bidirectionalflow, Integrated Gate Commutated Thyristors (IGCTs) are opposinglycoupled in parallel to the diodes in input rectifier section 520. In themodulator section 530, switching devices, such as high voltage SiCMOSFETS are utilized. The modulator section 530 in one example uses highfrequency switches such as SiC IGBT or a combination of SiC MOSFET/IGBT.Capacitors 525, 555 provide snubbing and high frequency filtering aregenerally not bulk capacitors used in DC bus applications. In thisexample, the high frequency transformer 540 downconverts the voltagethat is then processed by the second rectifier section 550. Therectifier components in the second rectifier section 550 are typicallyhigh frequency low voltage switches such as Si IGBT although they canalso be diodes or switching transistor components such as MOSFETs. Thesignal is then unfolded by the transistors in the unfolder section 560to provide the AC output 570. The low voltage low frequency switches ofthe unfolder 560 can be, for example, Si IGBTs.

In this particular embodiment, the power flow can be reversed with theAC output 570 becoming the AC input and the sections operating in areverse manner to provide an AC output at the original input 510. Asnoted herein, in one embodiment the switching devices in the modulatorsection 530 differ from those in the first rectifier section 520, thesecond rectifier section 550 and the unfolder section 560.

One of the features of the AC-AC conversion systems described hereinincludes using the systems as AC-AC building block. Referring to FIGS. 6a and 6 b, one example of a stacked single phase AC-AC converter system600 is shown. In this embodiment, several AC-AC electronic transformercells 610, 620, 630, 640 are electrically coupled to form a single-phasehigh voltage electronic transformer.

One embodiment configures the primary sections in series and thesecondary sections in parallel that enforces voltage sharing among cellsin series on the primary side and current sharing on the paralleledcells on the secondary side. Other configurations such as series coupledprimary sections and series coupled secondary sections, parallel coupledprimary sections and series coupled secondary sections as well asparallel coupled primary sections and parallel coupled secondarysections are further embodiments although active control may be requiredto ensure equal voltage and current sharing.

The electrical equivalent of the stacked converter system 600 is shownin FIG. 6( b) by the primary transformer 650 and secondary transformer660, which represents a large conventional transformer. For some highvoltage X kV that has four stacked electronic transformer cells 610,620, 630, 640, each of the corresponding series coupled primary windingshas ¼ of the high voltage X kV. On the electronic transformer secondaryside, the parallel-coupled windings produce a lower voltage Y kV. Thevariables X and Y are used to denote some arbitrary values forillustrative purposes.

One illustrative example is one of three phase 13.8 kVac input with a465 Vac output. The input to each primary portion of the electronictransformers carries ¼ of the 13.8 kVac. The parallel-coupled secondarywindings supply 465 Vac/(√3). It should be understood that the number ofstacked sections can be varied to provide for the desired operationalperformance. Likewise, the design of the assembly components can bedesigned according to a desired end result.

One embodiment of the high voltage high frequency inverter sectionincludes resonant or soft switching capability such that the transformerfunctions with softer edges. The soft switching feature in this exampleincludes additional components 680 such as a series coupled inductor andcapacitor. Soft switching components 680 such as an LC resonant tankcircuit can be incorporated in the other cells 620, 630, 640.

Yet an additional embodiment provides for redundancy wherein theconfiguration in FIG. 6 a depicts illustrates the switched cell 610 isbypassed by the bypass switches 685, 690, 695. In one example, there arefour cells 610, 620, 630, 640 that provide the three phase output. Onlythree of the cells 610, 620, 630, 640 are necessary to provide the threephase output wherein one or more of the cells 610 can include bypassswitching 685, 690, 695 such that if one of the other cells 620, 630,640 experienced some failure, the additional cell 610 can be included.The switching of the cells can also be arranged for efficiency such thatcertain cells are used for certain loads.

A three-phase electronic AC-AC converter or transformer is alsoconfigurable using the electronic transformer building blocks and can beconfigured, for example, as a three-phase delta-delta transformer suchas shown in FIG. 7 a.

In the example of FIG. 7 a, three single-phase electronic transformers715 are coupled to a three-phase high voltage input bus 705 that istransformed and output on a three-phase low voltage bus 710. In oneembodiment, each of the single-phase transformers has a first rectifiersection, inverter, high frequency transformer, second rectifier section,and unfolder section along with filtering capacitors. By coupling thetransformer sections 715 in this manner, a delta-delta electricalequivalent is constructed. For convenience, the individual components ofeach single phase transformer namely first rectifier section, inverter,high frequency transformer, second rectifier section, and unfoldersection can be referenced together as three-phase first invertersection, three-phase inverter, three-phase high frequency transformer,three-phase second rectifier section, and three-phase unfolder section.Signal processing in one embodiment is further described in FIGS. 10a-10 f.

A set of simulated waveforms is shown in FIG. 7 b to illustrate theprocessing steps by observing the signal waveforms. The basic processingsteps performed in each single-phase block are shown in the simulationwaveforms wherein a low switching frequency has been used in thissimulation to preserve clarity of representation of the waveforms.

There is a high voltage AC signal 720 that is rectified and a highvoltage rectified signal 725 is generated. The rectified signal 725 ismodulated into a high frequency link signal 730, such as a 20 kHzmodulated waveform. The high voltage and high frequency link signal isstepped down by the high frequency transformer to produce a low voltageand high frequency transformed signal 735 that is then rectified into arectified low voltage signal 740. Finally, the rectified signal 740 isunfolded into a low voltage AC signal 745. It should be noted that theterms high and low are relative terms and do not denote a specific valueunless otherwise specified. Filtering can be accomplished by capacitorsas detailed herein.

This approach basically uses the single-phase conversion blocks such asin FIG. 2. In one example, the front-end uses a simple diode rectifierwith a pulsed dc bus. One of the advantages of this approach is that allhigh frequency switching and controls are confined to one stage. Inaddition to increased reliability and efficiency, this allows the systemto be built with a minimum of high frequency switching devices.

FIG. 7 c illustrates a three-phase AC-AC converter system in accordancewith a delta-wye configuration that reduces peak currents in theswitching devices used in the electronic transformer. In delta-deltaconfiguration of FIG. 7 a the load currents are sinusoidal and theconnection to the electronics of the secondary between the phasesprovides control to shed currents to provide the output. One embodimentof signal processing is shown in FIGS. 10 d-10 f.

Unlike the delta-delta configuration of FIG. 7 a that is able tomanipulate the incoming waveforms, the delta-wye configuration of FIG. 7c requires a different scheme since the same current goes through theprimary and secondary and the manipulation between the phase currents isnot possible. Referring to FIG. 7 c the incoming AC signal 750 is asinusoidal waveform and is able to provide the output AC signal 760 withclean load currents. Additional components of the zero sequence orharmonic currents are extracted by the additional inverter 770 inaddition to the load currents.

In one example, if the load current is a sine wave having a certainfundamental frequency with current ‘a’, the zero sequence or thirdharmonic signal (additional component) has a frequency that is threetimes the fundamental frequency, with a current level that is about ⅓ ofthe current of the fundamental signal. At the primary side, the linecurrents become sinusoidal since the zero sequence currents are trappedin the primary delta connection.

It is well known that the transformer size is directly related to themagnetic flux, wherein the flux relates to an applied voltage across thewindings of the transformer over a certain time period. The integrationof the voltage multiplied by the time is the flux, namely, [Flux=∫Vdt].The transformer used for a particular application needs to handle thehighest flux condition along with any de-rating required for safeoperation. The highest flux level would typically be the largest voltagelevel over a certain time interval.

For example, for a fixed DC voltage, the flux is the summation of thevoltage over time, wherein the two parameters that dictate the fluxestablish the operating parameters. If the voltage levels are high, thenthe time interval needs to be short and vice-versa, if the time intervalis necessarily short, the voltage levels must be kept below a certainlevel. If not properly addressed, the flux increases until thetransformer saturates.

Referring to FIG. 8 a, FIG. 8 b, and FIG. 8 c, in conventionalapplications, the flux operation is controlled by the pulsating DC linkvoltage in conjunction with a constant switching frequency. With respectto the pulsating DC voltage the magnetic flux levels roughly flows theDC link signal such that when the DC link voltage is low, thecorresponding magnetic flux value integrated over the fixed timeinterval is also low. The upper and lower bounds of the magnetic fluxare fixed by certain physical parameters of the system. The switchingfrequency impacts the magnetic flux as the voltage is integrated overtime. And, the switching of the DC link signal changes the direction ofthe magnetic flux such that it oscillates between the upper and lowerlimits magnetic flux limits. The switching frequency has to be set toavoid saturation at the maximum values of the DC link signal such thatas the flux level approaches the positive saturation point, the voltagegoes negative, so the flux decreases thereby preventing saturation.Likewise, as the flux approaches saturation on the negative side, the DClink signal is reversed and the flux changes direction. There isconsiderable margin between the upper and lower magnetic flux limits(+/−1 Tesla in this example) and the magnetic flux values when the DClink voltage is low.

Referring to the constant switching frequency example, FIG. 8 a showsthe pulsed DC signal such as shown in FIG. 2 that is modulated by thehigh frequency modulating section to generate the voltage signal of FIG.8 b that is the voltage signal input to the transformer primary. FIG. 8c shows the transformer flux values and the margin 810 when the DC linkvalues are low.

Thus in order to keep the transformer core size smaller, the flux needsto be minimized. Increasing the switching frequency allows for thevoltage levels to be higher as they are integrated over a shorter timeperiod, thereby keeping the flux low. However, the high frequencyoperation incurs certain negative attributes related to the switchinglosses that impact the overall efficiency. And, for a constant switchingfrequency, for example 20 kHz such as accomplished by a chopper circuit,the switching frequency must be fast enough so that at the highestvoltage levels, the integrated voltage over the time period does notpermit the transformer to saturate.

Referring to FIG. 9 a, FIG. 9 b, and FIG. 9 c one embodimentillustrating a constant flux operation is depicted. In one example, atlower pulsed DC bus voltages, the switching frequency can be loweredthereby decreasing switching losses without increasing the transformerflux levels.

One embodiment of the present design varies the switching frequency suchthat there is a lower switching frequency when the voltage level is lowthereby having less switching losses during that period. As the voltagelevel increases, the switching frequency is increased in order tomaintain the flux within safe operations. Such an implementationprovides a relatively constant flux level that is kept within the safeoperating parameters. The time period T1 represents the time from thelowest magnetic flux level of FIG. 9 c to the highest magnetic fluxlevel. The time period T2 shows the smaller time that occurs as the DClink voltage approaches its maximum values and the magnetic flux valuesquickly achieves its upper and lower values. As previously described,upon reaching the lowest level, the bridge switches causing the DC linksignal to go from negative to positive and since the flux is directlyrelated to the voltage over time, the magnetic flux level in Figurerises from its lowest point to its uppermost point. As the magnetic fluxreaches its upper value, the switching causes the DC link signal to gonegative which thereby reduces the magnetic flux value.

Although there are physical upper and lower parameters, the upper andlower values for switching are design parameters and can be used tooptimize performance and efficiency. The margins are minimized oreliminated, as the magnetic flux levels are approximately constant.

The switching frequency can be based on the voltage levels such that theswitching frequency changes at particular levels. For illustrativepurposes, for the 100 Vdc pulsed signal, if a 20 kHz switching frequencyis required at the highest voltage levels in order to keep thetransformer from saturating, only a 10 kHz switching frequency isrequired at 50 Vdc. Likewise, at 25 Vdc, the switching frequency can be5 kHz.

In one example, an integrator can be used with some known or measuredvoltage level at some point with some associated scaling as necessary tocompute the flux. In another embodiment, a flux sensor can be integratedwith the core such that the flux level itself can be used to vary thefrequency. One example computes the variable switching frequency at eachswitching cycle while another example computes the variable switchingfrequency at some other time interval. An advantage of the variableswitching frequency is that there are lower switching losses in thetransformer and in the transistors.

One embodiment relates to maintaining constant flux operation, whereinat lower pulsed dc bus voltages the switching frequency can be loweredwithout increasing transformer flux levels.

A further feature of the systems detailed herein refers to the reductionof peak stresses. For three phase systems, the transformer primary sideto secondary side connections can be a coupled, for example, asdelta-delta, wye-wye, delta-wye and wye-delta configurations. Accordingto one embodiment, the peak current stresses in delta connected systemsor the peak voltage stresses in devices in wye-connected systems can bereduced by introducing a zero-sequence voltage scheme. One of theadvantages in reducing the peak values is that the individual componentsimplementing the scheme can have lower device ratings.

In one delta connected configuration, a load draws a sinusoidal currentand all the signals are sinusoidal at least external to the system.Inside the system, the phase currents flowing in the windings can bemanipulated such that the internal current can be lowered therebyallowing lower internal device ratings.

The conventional design of FIG. 10 a shows the internal phase currentsin the delta connected transformer. In this example each signal issinusoidal and has peaks at +/−100 amps.

As shown in FIG. 10 b, the individual line currents are obtained by thedifference of certain currents in the legs. In one example, Ia=Iab−Ica.

Referring to FIG. 10 c, the output line current are depicted, namely Ia,Ib, and Ic wherein they represent the processed phase currents, and thelevel of the line currents equals approximately 173 amps. The processingfor line current Ia as an example,Ia=Iab−Ica.

According to this example, the phase current waveform is manipulatedsuch as shown in FIG. 10 f wherein the waveform includes additionalcomponents. The additional components allow for a lower peak current,which in this example shows approximately +/−85 amps. The manipulatedwaveform can be generated by introducing a harmonic signal and propersequencing such that the line current output of FIG. 10 d is the same.For example, assuming a 60 Hz sine wave signal such as Ia depicted inFIG. 10 d. An additional component signal shown in FIG. 10 e is added tothe primary signal of FIG. 10 d. The additional component can be, forexample, a 180 Hz third harmonic with a current level about ⅓ of thelevel of FIG. 10 d. The primary signal of FIG. 10 d and the additionalcomponent of FIG. 10 e are combined with zero sequencing, and theresulting waveform combinations in FIG. 10 f that shows the altered thephase current outputs for Iab, Ibc and Ica. However the resulting linecurrents Ia, Ib and Ic from the altered phase currents is the samesignal shown in FIG. 10 c.

In a wye configuration, a similar processing can be performed bututilizing voltage waveforms such that the modified sine waves canreproduce the line voltages.

Thus one feature incorporates manipulating the internal waveforms tolower the peak stress values (current or voltage) so it ends up with thesame output waveform and output characteristics. One advantage ofincorporating this scheme is that the lower rated devices can handle thelower internal peak values. Another aspect introduces the zero-sequencevoltage in the primary line-neutral voltage to allow lower voltagedevices to be used for a certain system line-line voltage. For example,according to simulations, the device voltage ratings in one embodimentcan be reduced by about 15%.

FIG. 11 shows a representative flowchart of one of the embodiments forthe AC-AC conversion. Whether the AC-AC conversion is on a system orsubsystem level, it commences with receiving a high voltage AC signal1110, typically from a bus. The input signal may have some inputfiltering to better condition the signal for the processing.

The high voltage AC signal is rectified to generate the high voltagerectified signal 1120. The diode bridge detailed herein can be one meansfor rectifying the high voltage input AC signal. Optional filtering 1125can be employed to remove certain high frequency components.

Modulating the rectified high voltage signal 1130 produces a highfrequency high voltage modulated signal referred to as a high frequencylink signal. One example of the modulating frequency is 20 kHz, howeverthe modulation is not limited to a specific frequency and otherfrequencies can be used depending upon the input frequency, thecomponents and the intended output frequency.

The next step refers to transforming the high frequency links signalinto a lower voltage level 1140. A high frequency transformer can beutilized to step down the voltage level and the transforming istypically related to the design parameters of the transformer.

The low voltage modulated high frequency links signal is then rectified1150 and unfolded 1160 into a low voltage AC signal. Optional filtering1155 can be employed on the rectified signal to remove certain highfrequency components.

According to a one embodiment, the present invention includes theimplementation of various silicon carbide (SiC) devices, such as SiCMOSFET, SiC IGBT, SiC Schottky, PiN and JBS diodes. The overall systemcan be a mixed silicon-carbide (SiC) and silicon (Si) device topology.The SiC technology offer benefits, such as lower conduction andswitching losses, higher voltage and higher temperature capabilitiesthan their counterparts of Si devices and enable high density highfrequency, medium voltage solid state power subsystems. One embodimentincludes the power electronic transformer based on SiC wide-band-gapswitching devices.

By way of practical application, one embodiment for the electronictransformer detailed herein is for the integrated power systemarchitecture aboard ships and trains. The naval vessels used to employ asegregated ship service power bus typically fed by an auxiliary turbineand a propulsion bus typically fed by the main turbine. According tointegrated power system architectures, there is a common bus fed by themain and auxiliary turbines with appropriate conversion to feed the shipservice loads, weaponry, and the propulsion system. A common bus fed bythe multiple turbines provides for a flexible system with improvedefficiency. However, the power conversion resources require bulky andheavy transformers that may weigh several tons each to convert the highvoltage AC power to the loads. Ships have limited space and smallerfootprints for equipment are always preferred. In addition, gross weighttends to decrease efficiency and otherwise limit the weight that can becarried on board the ship. The replacement of conventionalline-frequency transformers with high frequency electronic transformersreduces the number of active switching stages and represents substantialreductions in size and weight of the distribution transformers in anelectric ship.

There are various commercial embodiments in which the electronictransformers detailed herein can be utilized. Wind energy is a cleancommercially viable alternative energy source and any cost andefficiency improvements are helpful. A majority of the current windturbines use a low voltage generator in the nacelle. The frequencyconverter and step-up transformer are located down-tower due to size andweight constraints and this requires multiple bulky low voltage, highcurrent cables to be run down the length of the tower. One solution isto place the frequency converter/transformer in the nacelle that allowslighter high voltage cables to be run down the tower.

Offshore/Subsea applications are another commercial usage involvingoffshore oil and gas platforms and subsea oilfields, where weight andspace constraints are important. For sub-sea oilfield applications, theelectrical equipment is often housed in pressurized sealed containers.Often heavy components such as transformers need to be supported onspecial pads and foundations and shrinking the size of transformers andother conversion equipment is very desirable.

Another application relates to utility distribution networksparticularly underground electrical distribution networks in dense urbanareas. Due to increased load demand, transformers placed in undergroundvaults need to be upgraded in ratings, but are sometimes constrained tofit in the existing vault space.

One of the features compared to other solid-state transformers is theminimization of high frequency switching stages. In addition to costreductions, the described systems alleviate problems of poorefficiencies afflicting many of the prior approaches. The describedtopology also uses a minimum number of switching devices, a feature thatis advantageous due to limited availability of these devices in theearly stages.

The electronic transformer provides significant benefits compared toconventional line-frequency transformers and the present high frequencydesigns. There is a significant size and weight reduction as well aslower cost due to the minimized number of active switching stages.Higher efficiency as well as lower cooling requirements are additionalbenefits.

Further features include lower Electro-Magnetic Interference (EMI)generation since high frequency switching is confined to one stage.There is also less control complexity and improved reliability.Switchgear requirements can be reduced. For example, controls can beused to limit fault currents or SCRs in the output stage can be used assolid-state breakers.

The foregoing description of the embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthis disclosure. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto.

What is claimed is:
 1. A single-phase AC-AC converter for an AC source,comprising: a first rectifier section rectifying the AC source into afirst pulsed DC link voltage signal; a high frequency modulating sectioncoupled to said first pulsed DC link voltage signal and producing a highfrequency AC voltage signal, wherein said high frequency AC voltagesignal is a modulated waveform having substantially similar amplitude tosaid first pulsed DC link voltage signal and having a varying switchingfrequency to maintain a relatively constant flux; a high frequencytransformer coupled to said high frequency AC voltage signal producing atransformed high frequency AC voltage signal; a second rectifier sectioncoupled to said transformed high frequency AC signal and producing asecond pulsed DC voltage signal; and an unfolder section coupled to saidsecond pulsed DC voltage signal and producing an output AC signal. 2.The AC-AC converter of claim 1 further comprising a reactive convertersection supplying a reactive current to said output AC signal.
 3. TheAC-AC converter of claim 2 wherein said reactive converter section iscoupled to said output AC signal by one of a parallel and seriesconnection.
 4. The AC-AC converter of claim 1 wherein said first andsecond rectifier section comprises antiparallel switches providingbidirectional and reactive power flow.
 5. The AC-AC converter of claim 1further comprising at least one additional series coupled electronictransformer forming a stacked electronic transformer.
 6. The AC-ACconverter of claim 1 further comprising at least one additional parallelcoupled electronic transformer forming a stacked electronic transformer.7. The AC-AC converter of claim 1 further comprising at least onecapacitor coupled between said first rectifier section and said highfrequency modulating section for filtering high frequency components. 8.The AC-AC converter of claim 1 further comprising at least one capacitorcoupled between said second rectifier section and said unfolder sectionfor removing high frequency components.
 9. The AC-AC converter of claim1 wherein said high frequency modulating section comprises an inverterwith at least two series coupled transistors to each switch position ofsaid high frequency transformer.
 10. The AC-AC converter of claim 9wherein at least one of said series coupled transistors has a differentvoltage rating.
 11. The AC-AC converter of claim 9 wherein said seriescoupled transistors are controlled such that at least one of saidtransistors is not toggled for at least some period of time.
 12. TheAC-AC converter of claim 11 wherein said series coupled transistors areswitched according to a threshold level selected from the groupconsisting of: only one of said transistors is toggled during a lowervoltage region, only one of said transistors is toggled during a middlevoltage range, and all of said transistors are toggled during peakvoltages.
 13. The AC-AC converter of claim 12 wherein said one of saidtransistors toggled during said lower voltage region is a lowervoltage-rated transistor with respect to said transistor toggled duringsaid middle voltage region.
 14. The AC-AC converter of claim 12 whereinsaid switching is based upon at least one of a measured voltage range ora calculated voltage range.
 15. The AC-AC converter of claim 1 whereinsaid high frequency transformer is one of a step-up transformer, astep-down transformer or a unity transformer.
 16. A single-phase AC-ACconverter comprising: a first rectifier section rectifying an AC sourceinto a first pulsed DC link voltage signal; an inverter section coupledto said first pulsed DC link voltage signal and producing a highfrequency AC signal, wherein said inverter section is comprised of atleast two transistor banks; a high frequency transformer coupled to saidhigh frequency AC signal producing a transformed high frequency ACsignal, wherein the high frequency AC signal (235) has substantiallysimilar amplitude to said first pulsed DC link voltage signal and isformed by applying the DC link voltage in alternating forward andreverse directions at a varying switching frequency to maintain arelatively constant flux; a second rectifier section coupled to saidtransformed high frequency AC signal and producing a second pulsed DCvoltage signal; and an unfolder section coupled to said second pulsed DCvoltage signal and producing an AC signal output at approximately a samefrequency as the AC source.
 17. The AC-AC converter of claim 16 whereineach transistor bank comprises at least two transistors coupled inseries.
 18. The AC-AC converter of claim 17 wherein at least one of saidtransistors has a different voltage rating.
 19. The AC-AC converter ofclaim 17 wherein said transistors in each said transistor bank areswitched such that at least one of said transistors is not toggled forat least some period of time.
 20. The AC-AC converter of claim 16further comprising at least one additional series coupled electronictransformer forming a stacked electronic transformer.
 21. A three phaseAC-AC converter, comprising: three single-phase electronic transformerscoupled together to convert a three phase AC signal input to a threephase AC signal output, each of said electronic transformers comprising:a first rectifier section rectifying said AC signal into a first pulsedDC link signal; an inverter section coupled to said first pulsed DC linksignal and producing a high frequency AC signal; a high frequencytransformer coupled to said high frequency AC signal producing atransformed high frequency AC voltage signal, wherein said highfrequency AC signal is a modulated waveform having substantially similaramplitude to said first pulsed DC link signal and having a varyingswitching frequency to maintain a relatively constant flux; a secondrectifier section coupled to said transformed high frequency AC signaland producing a second pulsed DC link signal; and an unfolder sectioncoupled to said second pulsed DC link signal; wherein said threeelectronic transformers produce said three phase AC signal output havinga frequency and shape similar to said three phase AC signal input. 22.The converter of claim 21 wherein each said electronic transformerproduces a phase current that is used to generate a line current,wherein each said phase current combines a harmonic component alteringsaid phase current, and wherein said line current remains the same. 23.The converter of claim 21 wherein said electronic transformers arecoupled to form one of a delta and wye connection on at least one of aprimary or a secondary.
 24. A method for converting AC power using asingle phase AC-AC converter section, comprising: receiving an input ACsignal; rectifying said input AC signal into a first pulsed DC linkvoltage signal; modulating the first pulsed DC link voltage signal intoa high frequency AC signal, wherein the high frequency AC signal (235)has substantially similar amplitude to said first pulsed DC link voltageand is formed by applying the DC link voltage in alternating forward andreverse directions at a varying switching frequency to maintain arelatively constant flux; transforming said high frequency link to atransformed voltage link; rectifying the transformed voltage link into asecond pulsed DC link voltage signal; and unfolding the second pulsed DClink voltage signal into an output AC voltage signal.
 25. The method ofclaim 24 further comprising introducing reactive currents into saidoutput AC signal.
 26. The method of claim 24 further comprisingproviding bidirectional and reactive power flow.
 27. The method of claim24 further comprising at least two of said AC-AC converter sectionscoupled together to accommodate a higher voltage AC source.
 28. Themethod of claim 24 wherein modulating said rectified AC signal comprisesswitching among at least two transistors coupled in series in eachswitch position.
 29. The method of claim 28 wherein said switching usesa lower voltage-rated transistor for lower voltages.